576 GLACIAL DEPOSITS IN SUBALPINE SWITZERLAND. [Aug. 1896, 



of bifurcation of the two arms of the Preglacial Rhine, 1 would 

 place the valley-floor at that point at contour 710, or 190 metres 

 higher than the present valley. As regards the Sihl valley, its 

 Preglacial course from the old confluence with the Linth near 

 "Wollerau to Schindelleggi and upward has already been dealt with 

 in a preceding part of this paper. 



In the profile (fig. 12), plotted from the various altitudes given 

 in the preceding paragraph, I have inserted at their respective levels- 

 the different low-level occurrences of Deekenschotter in the Zurich 

 valley between Turgi and the junction of the Preglacial Linth and 

 Sihl, near the old Ufenau bar. From the contours already given, it 

 is seen that the inclination of the ancient valley-floor is by no means- 

 arbitrarily chosen, but corresponds more or less closely to the 

 gradually ascending level of those Deekenschotter exposures, that 

 is, to their contact with the underlying solid rock, except the deposits 

 of Au and Altschloss, which, as already mentioned, lie in a zone of 

 subsequent subsidence. Moreover, having regard to the meandering 

 and irregular course of the river, the fall of 3*5 per kilometre of the 

 valley-floor corresponds to only about half, that is, to 1*75 kilometre, 

 or about 1 in 600 of the river, whose mean velocity, in a shallow 

 section of about 200 square metres, would not exceed \ metre 

 per second, or about 1 mile per hour. At this velocity the erosion 

 on the convex side of one bend of the river would be approximately 

 balanced by re-deposition of material on the concave side of the next. 

 Under these conditions the Linth, immediately before the first glacia- 

 tion, must have been essentially a sluggish river, the more so as its 

 course was not regulated as it is at the present day. Of such 

 ancient windings the Zurich valley offers numerous examples, one 

 of the most striking of which occurs at Wettingen and Baden, where 

 the river has repeatedly changed from one side of the valley to the 

 other, the present curved course being the exact reverse of a former 

 one, which I have indicated by a dotted line in the sketch-map 

 (fig. 2, p. 561). 2 



In the profile (fig. 12) of the Preglacial valley-floor, I have 

 further plotted (of course, only diagrammatically) the salient points of 

 the Molasse crest-line on both sides of the valley. This crest-line r 

 having been in many places buried under successive glacial deposits,, 

 which were in part again removed by subsequent denudation, has 

 not, since the advent of the Ice-age, undergone any very marked 

 changes as regards its general elevation, and therefore enables us T 

 in conjunction with the old valley-floor, to determine the con- 



1 The low alluvial saddle near Sargans, which now separates the Rhine from 

 the Walen lake-basin, is only 5 metres in depth and about 5 kilom. in width .- 

 In the year 1817 the Rhine nearly overflowed again into the Walen lake-basin. 

 In order to meet this contingency, the railway along that lake to Sargans and 

 Ragatz was not allowed to cross the bar by a cutting, but had to pass over it. 



2 Similar changes in the river-course were, no doubt, produced also in Glacial 

 times, when, during a recession of the glacier, the glacier-stream was banked by 

 the terminal moraine-wall, until it overflowed the latter at the lowest point off' 

 effected a breach at the point of least resistance. 



